PROJECT SUMMARY It is widely thought that identifying how neurons are connected to each other in a brain circuit, its wiring diagram, is a necessary step towards understanding how brain activity gives rise to behavior, and how it is perturbed by disease. Unfortunately, currently available methods have limitations that make it challenging to visualize these brain wiring diagrams. In addition, there is an urgent need in the field for a method that will make it possible not only to unveil brain connectivity, but also to genetically modify the functional properties of the neurons connected in a circuit. We recently developed a genetic system named TRACT and showed using Drosophila that it possesses both of these features. Unfortunately, many complex brain functions cannot be examined in Drosophila, and understanding them will require studying vertebrate animals. In recent years the zebrafish has emerged as a useful animal model to study complex brain processes, because it has a relatively simple yet conserved vertebrate brain, optical transparency during embryonic and larval stages of development, amenability to large-scale behavioral assays, the emergence of complex behaviors after only 5 days of development, and a growing suite of genetic tools that allow observation and manipulation of neuronal circuits in behaving animals. However, the usefulness of zebrafish for neuroscience research is constrained by a lack of methods to identify synaptically connected neurons. Here we propose proof-of-concept experiments to establish the TRACT system for use in zebrafish to identify the wiring diagrams of brain circuits, and to genetically manipulate the functions of neurons that mediate complex behavioral states such as sleep and arousal. These capabilities will broadly increase the usefulness of zebrafish as a model system to study vertebrate neuronal circuit function, both to reveal general principles of neuronal circuits that underlie specific behaviors, and to model complex human brain disorders such as autism, Alzheimer?s disease and schizophrenia.